Human society is releasing an increasing number of types and vast amounts of different complex organic compounds into the natural environment. Of these compounds, the emerging contaminants, those typically not regulated or routinely monitored by government agencies, include a wide range of pharmaceuticals and personal care products (PPCPs), and other compounds that cannot be degraded or removed in conventional drinking water and wastewater treatment processes. Many of these compounds can have adverse effects on the environment, animal, and human health (e.g., endocrine disruptors), even at low concentrations. For example, widespread release of antibiotics has led to the evolution of antibiotic resistant bacteria which reduce our capability to manage infectious diseases. Such compounds are released into drinking water, ground water, and wastewater from hospitals, water treatment plants, and distributed sources such as septic field and edge agricultural runoff. Many of these compounds are not readily biodegradable, some are highly persistent in the environment, some may accumulate in the food chain, and some may degrade into more hazardous compounds causing further environmental and health issues. Approximately 700 emerging pollutants, including their metabolic and degradation products, are listed in Europe.
The use of continuously flowing non-thermal plasma reactors to generate hydrogen peroxide and to degrade organic compounds has been investigated, and the following applications are incorporated by reference in their entireties: US Publication No. US 2017-0021326 A1 published on Jan. 26, 2017; U.S. Pat. No. 9,861,950 issued on Jan. 9, 2018. Plasma reactors are energy intensive and complete degradation of some organic compounds to mineralized products may not be economically feasible.
Biological reactors offer significant energy efficiency, but require significant residence times, on the order of days or weeks, in order to fully degrade some contaminants. Also, biological reactors are incapable of degrading some toxic organic compounds, or are incapable of completely degrading some organic contaminants to mineralized products. Plasma reactors that only treat liquid contaminants combined with biological reactors are incapable of degrading gas phase contaminants. Such combined systems also are not readily adaptable to changing contaminant composition streams.
A significant groundwater contaminant is 1,4-dioxane (dioxane, C4H8O2), which has attracted considerable attention in recent years. Co-contamination of 1,4-dioxane with 1,1,1-trichloroethane (TCA) and/or trichloroethene (TCE) is very common. As a probable human carcinogen and part of the U.S. Environmental Protection Agency (U.S. EPA)'s Unregulated Contaminant Monitoring Rule (UCMR3), U.S. EPA reported that dioxane was detected in about 19% of public water systems as of December 2013. Co-contamination of 1,4-dioxane with 1,1,1-trichloroethane (TCA) and/or trichloroethene (TCE) is common, since 1,4-dioxane was widely used as a stabilizer for chlorinated solvents (particularly TCA). The United States Air Force (USAF) Environmental Restoration Program Information Management System (ERPIMS) contains records for co-contamination of 1,4-dioxane, TCE, and TCA at 5788 groundwater-monitoring wells from 49 installations. It was reported that 94% of the groundwater monitoring wells showing TCE and/or TCA contained 1,4-dioxane. TCA and TCE have been regulated by U.S. EPA at maximum contaminant levels (MCL) of 0.2 and 0.005 mg/L, respectively, in drinking water (U.S. EPA). Several states in the U.S. have established drinking water and groundwater guidelines for 1.4-dioxane at 0.3-I pg/L (U.S. EPA).
Two types of technology—advanced oxidation processes (AOP) and biological processes—have been widely tested to treat dioxane, TCA, or TCE-contaminated groundwater, but their application is very limited due to the high cost of the AOP and slow reaction kinetics of the biological processes. To minimize the disadvantage of AOP and biological processes, a few efforts have been made to pretreat dioxane- or TCE-contaminated groundwater using the commonly used AOPs (H2O2+O3; H2O2+UV irradiation) to convert the contaminants into readily biodegradable compounds, and then further treat it using biological processes. The combined AOP and biological approach is not effective for TCA treatment due to its saturated molecular structure.
A variety of advanced oxidation processes (AOP) have been reported in the literature to treat 1,4-dioxane, TCE, and TCA. The commonly used processes combine either ozone (O3(aq)) or UV radiation with hydrogen peroxide (H2O2). Full-scale AOP treatment of dioxane or TCE has been reported in a few cases, however, use of AOP for TCA removal is much less since TCA, a saturated molecule, is much more difficult to oxidize than TCE. A comparison study showed that the amount of energy required to decompose TCA is 20-25 times higher than the energy required to decompose the same amount of TCE in a commonly used AOP. The primary limitation with the advanced oxidation processes is their energy intensive nature and high operation and maintenance costs. Most of the energy is used to completely mineralize the intermediates. Therefore, the majority of the energy is used to further remove the intermediates.
Compared to advanced oxidation processes, biological processes are generally much less expensive, but require much longer treatment time. The reported half-lives for dioxane, TCA, and TCE undergoing biodegradation in groundwater and surface water range from months to years.
The biodegradation rate of dioxane is very small due to the small maximum growth rate of the dioxane-degrading bacteria and the large half-maximum-rate constant of dioxane. The concentration of dioxane in groundwater is usually orders of magnitude smaller than the half-maximum-rate concentration of dioxane in Monod kinetics, leading to very small removal rate of dioxane. Therefore, to remove dioxane to below the drinking water guidelines (i.e., 0.3-1 μg/L), a very long treatment time is usually needed.
Current experimental data suggest that dioxane can only be biologically removed under aerobic conditions. Under aerobic conditions, TCA and TCE can be biologically oxidized to CO2, H2O, and Cl− (chloride) through a co-metabolic pathway. In the aerobic case, the organisms grow on another substrate (i.e., a primary substrate such as methane, ethene, propane and acetate) and the enzymes induced under the particular growth conditions fortuitously biodegrade TCA and TCE. The co-metabolic degradation rate is usually very slow, leading to a long treatment time.